Along with performance improvement of electronic devices, heat generation thereof has been increased as well. If the heat is left untreated, it invites a situation in which the failure of the device, performance deterioration, shortened life time and so on. Therefore, the device has to be equipped with a cooling function. Generally, a fan is used for the cooling function. Examples of the means for enhancing the cooling performance of a fan include increasing the number of revolutions (revolving speed) of the fan and increasing the number of mounted fans. For example, high cooling performance is required for a countermeasure against increased heat generation caused along with high-density mounting in a disk array apparatus, and thus a cooling method using a high-output fan is employed therefor.
However, the fan used in cooling of a heat-generating electronic device and the like is the major generation source of noise in the electronic device (for example, disk array apparatus). A simple means or countermeasure for suppressing or insulating noise of the fan is, for example, to provide an additional object or equipment for a countermeasure against noise such as a sound insulation wall in an object matter to which the fan is provided.
In any of the above-described methods for enhancing the cooling performance, noise is increased. In other words, the cooling performance and silent (noise reduction) performance conflict with each other. When the cooling performance is enhanced, noise is increased; and, when the silent performance is increased, the heat cannot be readily cooled.
As a conventional technique for eliminating or suppressing noise of the fan, for example, a technique of eliminating or suppressing noise (generated-sound) by transmitting a waveform of an opposite (reversed) phase with respect to the noise of the fan by a speaker provided separately from the fan and cancelling out and merging (mixing) the waveforms is known as so-called active noise reduction (ANR).
As a conventional technique of a fan and ANR is described in Japanese Patent Application Laid-Open Publication No. 6-508695 (Patent Document 1). All the techniques disclosed in this document are the type that carries out active noise reduction by a speaker mounted separately from the fan, and this type is conceivably invented for reducing the noise of a large fan. However, this method is difficult to be realized in the electronic devices in which downsizing and increase in density is notable. Generally, a large dimensional restriction is imposed on an electronic device by an aspect of functions such as requirements for space saving and rack mounting. Therefore, in many cases, the fan is mounted on an inside of back end of a chassis of the device. Also, there is a requirement in terms of design that the opening area is desired to be designed large in order to enhance exhaust capability (cooling capability) as much as possible. With respect to this, the above-described conventional technique has a problem that the opening area has to be reduced when the speaker is set in the vertical direction of the fan, and dimensional restrictions are large when the speaker is set in the front/rear direction of the fan.
Further, a conventional technique in an equipment field other than the above-described conventional technique is described in Japanese Patent Application Laid-Open Publication No. 2005-76585 (Patent Document 2). The technique disclosed in this document employs a method for reducing noise by vibrating a fan installed in a duct of a ventilation hole by an active-type magnetic bearing. However, this is a large-scale technique in which magnetic bearing units are disposed at front and rear of the fan and cannot be applied to general electronic devices as well as the above described conventional technique.
Furthermore, normally, as a usage system of a fan in an electronic device, not the method that obtains high torque/high output by increasing the size of the fan alone and feeding a large current but the type that carries out cooling by a combination of a plurality of small fans of medium/low output is popular in the case where, for example, the cooling performance with respect to equipment is deficient. One reason thereof is to ensure redundancy against a fan failure, and another reason thereof is to avoid the influence on cables, connectors, power source, etc. since the power source voltage (DC voltage) by which the electronic device is operated with low voltage, thus a large current is needed compared with the fan output power.